Ultrasonic electronic lens with reduced delay range

ABSTRACT

Ultrasonic reflections are received by a transducer array whose resultant signals are passed through an array of time-varying delay lines to provide focusing at all depths. The delay lines are made up of a combination of two components, one increasing and one decreasing with the distance to the axis of the array. This combination minimizes the range of delay variations required over the depth range. When using charge coupled devices as delay elements, this combination minimizes the required range of clock frequencies.

The invention described herein was made in the course of work under agrant from the Department of Health, Education, and Welfare.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to ultrasonic imaging systems. In a primaryapplication the invention relates to dynamically-focused receiversystems using arrays of transducers and time-varying delay lines.

2. Description of Prior Art

Ultrasonic imaging techniques are widely used for non-destructivetesting in industry and for visualizing soft tissue structures inmedical diagnosis. One of the principal difficulties with the existinginstruments stems from the use of non-focused patterns in the transmitand receive modes. Thus the lateral resolution is determined primarilyby the transducer size and by diffraction spreading. This results in afundamental compromise in the size of the transducer aperture. A largetransducer results in a uniform collimated beam pattern with relativelypoor lateral resolution throughout the depth of the object. A smalltransducer results in good lateral resolution at small depths but, dueto significant diffraction spreading, results in significantdeterioration at the longer depths. Most transducer apertures are aboutone cm. in extent as a resolution compromise.

In an effort to provide improved lateral resolution in B scan imaging,lens systems are often used which improve the lateral resolution at theregion of focus but not elsewhere. When using transducer arrays,electronic focusing can be accomplished by dynamically varying the delayapplied to each transducer signal as the ultrasonic wave propagates. Onesuch system has been described by G. Kossoff in "An Historical Review ofUltrasonic Investigations at the National Acoustic Laboratories," J.Clinical Ultrasound, Vol. 3, p. 39, March 1975. In one of his systemsDr. Kossoff uses a concentric ring array with time-varying delay linesconnected to each ring to provide dynamic focus. This was also describedat the meeting of the American Association for Ultrasound in Medicine inSeattle, Washington in October 1974. This system is not capable ofelectronic deflection and is therefore mechanically scanned to produce atwo-dimensional image. A system using a linear transducer array withelectronic deflection and focusing was described by F. L. Thurstone in"A New Ultrasound Imaging Technique Employing Two-Dimensional ElectronicBeam Steering," Acoustical Holography, Vol. 5, Plenum Press. Hereswitched delay lines are connected to each transducer output and used toprovide beam deflection and dynamic focus.

In both of these systems the delay elements represent a significantproblem. They are noisy, difficult to rapidly switch, bulky, and requirea relatively large delay range. In order to solve many of these problemsthe CCD (charge coupled device) is used as the analog variable delayelement because of its small size and simplicity. It makes possible ahand-held ultrasonic probe, which is important to physicians. The use ofCCD's in this dynamic focusing application is described by J. Shott andR. Melen in "The Rozorback CCD: A High-Performance Parallel Input DelayLine Architecture," Proc. of 1975 Int. Solid State Circuits Conf., pp.150-151, February 1975. Although the CCD delay elements solve many ofthe problems, the large depth range required in ultrasonic imagingsystems requires a correspondingly large range in clock frequencies.Since the lowest clock frequency is set by the signal sampling rate, thehighest frequency can become greater than the capability of CCDtechnology.

Another system using variable delay elements for dynamic focusing isdescribed in U.S. Pat. No. 3,918,024 issued to A. Macovski, entitled,"Ultrasonic Array for Reflection Imaging." In this system a weightedring array is used as the transmitter to provide good resolution in onedimension at all depths. The electronic focus is accomplished bytime-varying delay lines such as CCD's with controlled clockfrequencies. These delay lines, however, require a large range in orderto focus over the desired depth range. This range, as previouslymentioned, may well be beyond that presently available with CCD's. Inaddition to electronic focusing, this patent described methods toprovide electronic deflection by using an array of controlled delayelements which are changed following each scan line so as to provide asector scan.

The present invention is directed toward reducing the delay range forany variable delay system, including the CCD's. A paper based on theapplication of this invention to reducing the delay range using CCD'sis, "CCD Dynamically Focused Lenses for Ultrasonic Imaging Systems," byR. D. Melen, J. D. Schott, J. T. Walker and J. D. Meindl, Proc. of theInternational Conference on Applications of CCD, San Diego, Oct. 29-31,1975.

SUMMARY OF THE INVENTION

An object of this invention is to achieve dynamic focusing over theentire depth of interest in ultrasonic imaging systems. A further objectof this invention is to minimize the relative variations in delayrequired in dynamically-focused ultrasonic imaging systems.

Briefly, in accordance with the invention, an array of ultrasonicsignals are passed through a time-varying delay system. This systemconsists of two delay structures, one increasing and the otherdecreasing in delay with the distance to the axis. When either or bothof these delay structures are varied in time, only relatively smallchanges are required to achieve focusing, thus minimizing the relativedelay range required. When using CCD's, this represents a relativelysmall change in clock frequency. Alternatively the delay system caninvolve a lens and a time-varying delay structure to achieve the sameresult.

BRIEF DESCRIPTION OF THE DRAWINGS

For a more complete disclosure of the invention, reference may be madeto the following detailed description of several illustrativeembodiments thereof which is given in conjunction with the accompanyingdrawings, of which:

FIG. 1 is a block diagram illustrating an embodiment of the inventionusing a linear transducer array;

FIG. 2 illustrates an embodiment using electronic deflection andfocusing;

FIG. 3 illustrates an embodiment using a concentric annular ringtransducer array; and

FIG. 4 illustrates an embodiment using an acoustic lens.

DESCRIPTION OF THE PREFERRED EMBODIMENT

In ultrasonic A mode and B mode imaging systems a pulsed wavefront ispropagated through the object of interest and a sequence of reflectionsare received. Since the velocity of propagation is known within a smallvariation, the time each reflection is received substantially representsits depth. One of the primary problems in these systems is that oflateral resolution. Since the wavelengths λ used, of about 0.5 to 1.0mm, are comparable to the resolution of interest, the resolutionconsiderations are dominated by diffraction. When ultrasonic reflectionsare received by a transducer of lateral dimension D, the resolution sizein the near field region, a distance of approximately D² /λ from thetransducer, is essentially that of the transducer size itself, thedistance D. Appreciably beyond D² /λ, the beam diverges with an angle ofabout λ/D so that the resolution is approximately (λ/D)z where z is thedpeth. Thus a larger transducer deteriorates the near field resolutionand helps the far field resolution and vice versa. Most commercialinstruments use approximately a 1 cm. transducer which provides anear-field collimated beam for most of the depth of interest in thebody.

The resolution can be optimized at specific planes by using a focusinglens. The lens is capable of focusing in the near field region toproduce a resolution at the focal plane of (λ/D)f where f is the focallength of the lens. In this case the resolution is improved withincreasing the transducer size D. Unfortunately, however, this smallresolution size is achieved in one depth region only, with all otherdepths having a significantly larger pattern. Since ultrasonic wavestravel at relatively low velocities of 1500 meters/sec in water, anelectronic focusing system can be used to dynamically focus at eachdepth plane of the propagating pulse so that diffraction-limited lateralresolution is always achieved.

Electronic focusing is accomplished using an array of ultrasonictransducers which receive the reflections from a region at differenttimes because of the differences in path length to each transducer. Anarray of delay elements are used on the transducer signals to compensatefor the differences in propagation time. The delay-line outputs aresummed in an adder to produce a focused signal which represents equaltotal delay from the depth plane of interest to the adder. If the delaylines are varied in time the focusing can be accomplished at all depthsas the pulse propagates.

Referring to FIG. 1, a linear array 13 is used for providing dynamicfocus in one dimension. For simplicity only four elements, 14-17, areshown in the array. Initially object 10 is insonified with a propagatingultrasonic pulse. This can be accomplished by using the transducers ofarray 13 as a transmitting array by closing multipole switch 19. Thisconnects all of the array elements to transmit pulse generator 18. Apulsed ultrasonic wave will begin propagating through the object havinglateral dimensions of approximately the size of the array. The waveexcites each of the reflective surfaces in object 10. Reflectivesurfaces 11 and 12 are shown as examples. The round trip propagationtime from each transducer to the on-axis reflecting surfaces and back isgiven by

    T.sub.p = 2r/c = (2/c)√x.sup.2 + z.sup.2

where r is the distance from each transducer to the on-axis reflector, zis the depth of the reflector, x is the coordinate along the transducerand c is the velocity of propagation. The transducer is followed by anelectrical delay line having, for each z plane, a delay of T₀ - (T_(p) -(2z/c)), where T₀ is a fixed delay and 2z/c is the delay of the centralelement where x = 0. Thus the total delay of each reflected wave fromreflector to the transducer to the adder 23 is T₀ + (2z/c). This equaldelay at each depth provides focusing for signals on the axis at planez. Reflections from other lateral positions will arrive at differingtimes in the adder so that their relative phases will add destructivelyand thus provide a low amplitude at the output. The 2z/c portion of thetotal delay represents the different times at which signals fromdifferent depths are received. The purpose of the focusing delaymechanism is to equalize the delay from the different receivedtransducer signals from each reflection.

Since the reflections of interest usually make a relatively small anglewith the axis of the array, we can use the paraxial approximation whereT_(p) is given by ##EQU1## The first term, 2z/c, is the exact value ofthe round-trip time to the center element. It is this time which definesthe depth plane of each reflection. The second term, x² /zc,approximates the differences in the propagation time from the reflectorto each transducer at position x. Thus the propagation time differencesincrease approximately as the square of the distance from eachtransducer to the center or axis of the array. If a more accuraterepresentation of T_(p) is used, for reflections received at steepangles, additional terms must be used including higher even powers of xsuch as x⁴ and x⁶. In general, however, the differences in propagationtime increase as the magnitude of the distance to the transducer axis.

To focus on a reflection from a given depth z a set of delay elementsare used which compensate for the delay differences, x² /zc, so that thereflections from each transducer add in phase. Since a given fixed delayin each signal path does not affect the focusing, the delay applied toeach transducer signal is made T₀ - (x² /zc). In that way the totaldelay from a reflection to the transducers and through the array offocusing delay elements are each T₀ + (2z/c).

Thus, referring to FIG. 1 and ignoring the clocks 25 and 26 and delaylines 21, if the delay lines 22 each had values T₀ - (x² /zc), thesignals applied to adder 23, originating from depth z, would add inphase and produce a focused signal which is applied to display 24. Inaddition if these delay lines 22 were made time varying so as to havethe required T₀ - (x² /zc) delay at each depth, as the transmittedultrasonic wavefront propagates, focusing would be achieved at alldepths.

The difficulty with this arrangement is in the large delay variationwhich must be achieved. The delay change, ΔT, is given by

    ΔT = (x.sup.2 /z.sub.min c) - (x.sup.2 /z.sub.min c)

In ultrasonic imaging z_(min) is approximately 1.0 cm and z_(max)approximately 25 cm. The difficulty of achieving this large variation indelay depends on the type of delay line used. In any case the problem iscomplicated by the fact that the delay line must also pass therelatively high ultrasonic frequencies in the 2-3 mhz range. In modernintegrated circuit technology Charge Coupled Devices (CCD's) areexcellent candidates for variable delay lines. In these devices a chargepacket, representing the analog signal, is moved along at a ratedetermined by the clock frequency. The minimum clock frequency must beat least twice the highest signal frequency as dictated by the samplingtheorem. This frequency would then determine the maximum delaycorresponding to the closest depth plane z_(min). The minimum delay isreached by using a very high clock frequency. If a frequency of about5.0 mhz is used for the maximum delay to satisfy the samplingrequirement, a frequency approaching 100 mhz might be required for theminimum delay corresponding to the furthest depth plane. This is beyondthe state-of-the-art in CCD technology.

The solution to the problem is the use of two arrays of delay structureswith opposing variations of delay with transducer position as shown inFIG. 1. The first focusing array of delay structures 21 consists of anarray of lines whose delay variation increases approximately as thesquare of the distance of each transducer to the axis of the array. Thusthe transducer signals from transducers 15 and 16 have less delay indelay-line array 21 than the signals from transducers 14 and 17.Although these delay line elements can represent a variety ofstructures, in FIG. 1 they are indicated as CCD delay lines driven byclock generator 25. Delay array 21 is followed by a second focusingarray of delay structures 22 which is driven by clock generator 26. Inthis delay array the delay variation decreases approximately as thesquare of the distance to the axis of the transducer array. Thus thedelay used for the signals from transducers 15 and 16 is larger thanthat of transducers 14 and 17. In this general way relatively smallchanges in delay are required in delay line arrays 21 or 22 in order toprovide the desired variation in delay with transducer position. Insystems using CCD's, this means a relatively small change in clockfrequency is required, a very important parameter for the use of CCD's.The outputs of both delay systems 21 and 22 have the delays equalizedfrom the ultrasonic reflections. When they are added in adder 23 theyadd in phase from the depth corresponding to the propagation time of theultrasonic wave. A focused signal is formed representing that depthplane and having the desired lateral resolution properties of a focusedsystem. This signal is displayed in display 24 in the conventionalB-mode display.

In general delay lines 21 have delays of k₁ x² and delay lines 22 have adelay of T₀ - k₂ x² where x is, as before, the distance from eachtransducer to the axis of the array and k₁ and k₂ are the parameters ofeach delay array. T₀ must be at least equal to k₂ x_(max) ². The totaldelay is thus T₀ + (k₁ - k₂)x². If k₁ and k₂ are made relatively large,they each have to be fractionally changed only slightly for theirdifference to have the desired range of (x² /z_(min) c) - (x² /z_(max)c). Either or both of lines 21 and 22 can be time-varying. To achievethe maximum delay variation they would each be time-varying in theopposite sense. Thus, for the minimum depth, which requires the greatestdelay difference, k₁ would decrease and k₂ would increase. If lines 21and 22 were CCD's, as the transmitted pulse begins to propagate clock 26would be at its lowest frequency and clock 25 at its highest frequencyto provide the maximum decrease of delay with distance to the axis. Asthe pulse propagates toward greater depths the frequency differencebetween 26 and 25 would decrease until they approach each other forinfinite depths. Using appropriately long CCD's clock frequencyvariations of a few percent can provide the entire delay range ratherthan the variation of greater than an order of magnitude which isotherwise required.

Similar results can be obtained with either 21 or 22 a fixed array ofdelay lines and the other time-varying. Here again (k₁ - k₂)x² can bemade to have a large variation while one of them, k₁ or k₂, has arelatively small variation. In FIG. 1 the delay elements which increasedin delay with distance to the axis 21 preceded those which decreased 22.These can be reversed in sequence with no change in the system.

In order to achieve a B-scan the transducer array 13 of FIG. 1 wouldhave to be mechanically scanned along the object to create a sequence oflines representing the reflectivity of the object. These lines aredisplayed on display 24 whose slow scan is synchronized with the motionof the array. To create a real-time electronic system, in addition todynamic focus, a system of electronic deflection is required as shown inFIG. 2. Here an additional array of deflecting delay elements 30 is usedto provide steering of the beam so that a sector scan is created witheach scan line at a slightly different angle. The reflection from apoint x₀ from the axis, at a depth z, have a propagation time given by##EQU2## To compensate for this propagation time using an array we mustdeal with those components which vary with the transducer coordinate x.The x² /2z term is the quadratic focusing term previously described. The2xx₀ /zc is the new deflection term. If an array of delays having thisvariation is used, the array will essentially be pointed at an angle oftan⁻¹ (x₀ /z). Notice that these delays are proportional to x so thatthey increase with positive x values and decrease with negative xvalues. The other two terms in T_(p), 2z/c + x₀ ² /zc, represent thepropagation time from the target and are independent of the transducercoordinate x.

Deflection delay elements 30 are switched following each round trip tochange the angle of the scan from the origin. Unlike the focusing delays21 and 22 they need not be changed during the propagation interval. Theycan be switched fixed delay lines or CCD's whose clock frequency ischanged for each complete line scan. Since they vary linearly with x,for an on-axis scan all the lines in 30 would have an equal delay T₁.For deflection, delay element 32 would be switched to T₁ + Δ, 31 to T₁ +2Δ, 33 to T₁ - Δ, and 34 to T₁ - 2Δ where Δ determines the angle ofdeflection. For small angles Δ is given by

    Δ = (2xθ/c)

where θ is the deflection angle.

Following deflection the system can be electronically focused exactly asin FIG. 1 using focusing delay lines 21 and 22. FIG. 2 shows twovariations on these focusing systems. Since the focusing delayrequirement is an even function, the same focusing delay requirementsexist for transducer elements having the same distance to the transduceraxis. Thus focusing delay 21, as shown in FIG. 2, can be made with halfthe number of delay elements by first tying the transducer signals fromtransducers 14 and 17 together and 15 and 16 together since they requirethe same delay. As shown, focusing delay system 21 is then reduced tohalf the number of elements.

Focusing delay lines 22 can be constructed in the identical fashion,with the opposite variation of delay, and the outputs applied to anadder. As an alternate arrangement, focusing delay lines 22 and 23 inFIG. 1 can be replaced by a focusing delay line 36 shown in FIG. 2. Inthe tapped delay line, the signal requiring the longest delay is placedat the sending end of the delay line, with the signal requiring theshortest delay placed at tap 36 nearest the terminating end of the delayline 35. Intermediate taps are used for the intermediate transducersignals as required. This construction reduces the number of delayelements used and automatically provides the adding function. Delaylines 21 and tapped delay line 35 in FIG. 2 can be made of any type ofdelay structures including CCD's. Either one or both can be madetime-varying as previously indicated. Although they are shown followingthe deflection delay system in FIG. 2, this same focusing system can beused in the non-deflected system of FIG. 1.

The systems described thusfar have provided focusing in one dimensiononly. FIG. 3 illustrates a system for focusing in both dimensions usinga concentric-ring transducer array 40. The transmit system is not shown,although the transmit system of FIG. 1 can be used. The same set offocusing delay lines 22 and 21, having opposite variations with radius,are connected to the transducer signals at each annular ring. These canboth be time-varying CCD's controlled by clocks 25 and 26, or one delaysystem can be fixed with the other time varying. This system does notprovide electronic deflection and would be manually scanned. Theelectronic focusing would provide good lateral resolution in alldimensions.

Two-dimensional focusing can also be achieved by using the systems ofFIGS. 1 and 2 in the receive mode only and using a transmitter arraypattern which provides focusing in the orthogonal direction. One methodof doing this is the use of a ring or annular transmitter array such asthat of the previously described U.S. Pat. No. 3,918,024. In this systemthe transmitted beam provides good resolution, in one lateral dimension,at all depths. The dynamically focused linear array provides focusing inthe other dimension with the overall product of the patterns having thedesired two-dimensional focusing at all depths.

To avoid the complexity of two focusing delay systems, the system inFIG. 4 can be used. Here the delay compensation required for focusing isapportioned before and after transducer array 13. A lens 51 is usedbefore the transducer array 13 to provide increasing time delay, withdistance to the transducer axis, for the ultrasonic reflections.Following the transducer array 13 the transducer signals are subjectedto decreasing time delay, with distance to the transducer axis, usingfocusing delay lines 22. The delay line outputs are added in 23, asbefore, to form a focused signal which is displayed on display 24. Thissame configuration can use the deflection delay lines 30 of FIG. 2 toobtain a real-time sector scan. Also the array of focusing delay lines22 can be simplified by using the transducer signals in pairs as in 21in FIG. 2 or by using a tapped delay line as in 35 in FIG. 2 toaccomplish both the delay and adder functions. If the focusing delaylines 22 are CCD's driven by clock 26 the system would have theadvantage that only a single variable-frequency clock is used to controlthe depth of focusing. In this way undesired beat frequencies betweenclock frequencies are avoided.

In the lens system, lens 51 is normally a solid material, such aslucite, whose velocity of propagation c₁ is normally greater than c,that of water. Thus the convex lens shown becomes a negative lens,unlike the standard case in optics. A negative lens has an increasingtime delay with radius. The relative time delay T of various radii ralong the lens compared to that of the axis is given by ##EQU3## where Ris the radius of curvature of lens 51. Thus with c₁ > c, the delaydecreases with the square of the radius of transducer distance to theaxis. The lens 51 is housed in water-tight housing 50 which containsfluid 52 which is generally water and flexible coupling membrane 53.This membrane conforms to the contours of object 10 to assure goodcontact. Since the object 10 is normally soft-tissue, it has the samevelocity properties as the water 52 and thus does not distort the lensaction.

Although the system of FIG. 4 utilized a negative lens and a focuseddelay-line array which decreased with the square of the distance to theaxis, the opposite arrangement can also be used. The lens system can bemade positive by either using a concave structure for solid lens 51, orcontinuing to use a convex lens but with a velocity c₁ less than c.Velocities less than that of water can be achieved with various liquids.These would be enclosed by a membrane to provide the convex shape. Witha positive lens, focused delay-line array 22 would then provideincreased delay with the sequence of the distance to the axis.

What is claimed is:
 1. Apparatus for receiving ultrasonic reflectionsfrom an object comprising:a source of pulsed ultrasonic radiation whichinsonifies the object and produces a sequence of ultrasonic reflections;a transducer array for receiving the sequence of ultrasonic reflectionsand producing a plurality of transducer signals; a first focusing arrayof delay structures whose inputs are connected to the transducer signalsand whose delay varies with the magnitude of the distance of eachtransducer to the axis of the array; a second focusing array of delaystructurs whose inputs are connected to the outputs of the firstfocusing array of delay structures and whose delay varies with themagnitude of the distance of each transducer to the axis of the array ina manner opposite to that of the first focusing array of delaystructures so that the sum of the delays from each transducer throughthe first and second focusing delay structures decreases with themagnitude of the distance to the axis of the array; means for adding theoutputs of the second focusing array of delay structures to produce afocused signal; and means for utilizing the focused signal.
 2. Apparatusas recited in claim 1 wherein the first focusing delay array istime-varying and the second focusing delay array is fixed so that thetotal delay difference from each transducer substantially compensatesfor the propagation delay difference from each ultrasonic reflection toeach transducer whereby focusing is achieved.
 3. Apparatus as recited inclaim 1 wherein the first focusing delay array is fixed and the secondfocusing delay array is time-varying so that the total delay differencefrom each transducer substantially compensates for the propagation delaydifference from each ultrasonic reflection to each transducer focusingis achieved.
 4. Apparatus as recited in claim 1 wherein the firstfocusing delay array is time-varying and the second focusing delay arrayis time-varying in a manner opposite to that of the first so that thetotal delay difference from each transducer substantially compensatesfor the propagation delay difference from each ultrasonic reflection toeach transducer whereby focusing is achieved.
 5. Apparatus as recited inclaim 1 wherein the first focusing array of delay structures has itsdelays increasing substantially as the square of the distance of eachtransducer to the axis of the array and the second focusing array ofdelay structures has its delay decreasing substantially as the square ofthe distance of each transducer to the axis of the array.
 6. Apparatusas recited in claim 1 wherein the first focusing array of delaystructures has its delays decreasing substantially as the square of thedistance of each transducer to the axis of the array and the secondfocusing array of delay structures has its delays increasingsubstantially as the square of the distance of each transducer to theaxis of the array.
 7. Apparatus as recited in claim 1 wherein thetransducer signals from all transducers having the same magnitude of thedistance to the axis of the array are connected together whereby thefirst and second focusing arrays of delay structures require fewer delaystructures.
 8. Apparatus as recited in claim 1 including a deflectingarray of delay structures, whose delay varies in amount and in polaritysubstantially as the distance to the axis of the array, connectedbetween the transducer signals and the means for adding.
 9. Apparatus asrecited in claim 1 wherein the second focusing array of delay structuresand the means for adding consists of a tapped delay line wherein thetaps are the inputs and the focused signal appears at the output. 10.Apparatus for receiving ultrasonic reflections from an objectcomprising:a source of pulsed ultrasonic radiation which insonifies theobject and produces a sequence of ultrasonic reflections; a transducerarray for receiving the ultrasonic reflections and producing a pluralityof transducer signals; a lens positioned adjacent to the transducerarray, between the object and the transducer array, whose delay of theultrasonic reflections varies with the magnitude of the distance of eachtransducer to the axis of the array; a focusing array of time-varyingdelay structures whose inputs are connected to the transducer signalsand whose delay varies with the magnitude of the distance of eachtransducer to the axis of the array in a manner opposite to that of thelens so that the delay difference of each transducer signal compensatesfor the delay difference from each ultrasonic reflection, through thelens, to each transducer; means for adding the outputs of the focusingarray of delay structures to produce a focused signal; and means forutilizing the focused signal.
 11. Apparatus as recited in claim 10wherein the lens is a positive lens whose delay of the ultrasonicreflections decreases with the magnitude of the distance of eachtransducer to the axis of the array and the delays of the focusing arrayof time-varying delay structures increases with the magnitude of thedistance of each transducer to the axis of the array.
 12. Apparatus asrecited in claim 10 wherein the lens is a negative lens whose delay ofthe ultrasonic reflections increases with the magnitude of the distanceof each transducer to the axis of the array and the delays of thefocusing array of time-varying delay structures decreases with themagnitude of the distance of each transducer to the axis of the array.13. Apparatus as recited in claim 10 wherein the variation of delay ofboth the lens and the focusing array of time-varying delay structures issubstantially proportional to the square of the distance of eachtransducer to the axis of the array.
 14. Apparatus as recited in claim10 wherein the focusing array of time-varying delay structures and themeans for adding consists of a tapped time-varying delay line whereinthe taps are the inputs and the focused signal appears at the output.15. Apparatus as recited in claim 10 wherein the transducer signals fromall transducers having the same magnitude of the distance to the axis ofthe array are connected together whereby the focusing array oftime-varying delay structures has a reduced number of structures. 16.Apparatus as recited in claim 10 including a deflecting array of delaystructures, whose delay varies in amount and in polarity substantiallyas the distance to the axis of the array, connected between thetransducer signals and the means for adding.